To “run what you brung” is an old racing term meaning racing what you rode to the race.
Back in the late 1940s when servicemen came back from the war. They had no money for racing bikes, but they wanted to have fun. Hooligan racing was mostly a Southern California thing where they could race year round. People would ride a motorcycle to the event, race with it, and then ride the same bike home. - Bruce Brown on Hooligan racing
To me, run what you brung can be about both competing with your daily whip but also being ready to take on challenges without extra preparation. In other words, not hesitating to go for it with what you showed up with. This is better than “fake it till you make it” because you’re not faking. You’ve done this. You’ve got this.
You’re doing what you’ve done before but only faster or with higher stakes. How confident are you to compete armed with what you currently have on hand? How do you ensure that what you’ve brung will be enough?